An X-ray tube assembly included in an X-ray diagnostic apparatus generates X-rays by causing an electric current to flow through a filament to emit thermal electrons and causing the thermal electrons to hit a target. The characteristics of the X-rays depend on a tube current flowing between the filament and the target. The tube current depends on the number of thermal electrons emitted from the filament. The number of thermal electrons emitted from the filament depends on a temperature of the filament. The temperature of the filament depends on a resistance of the filament and an electric current flowing through the filament. Therefore, the X-ray tube assembly changes the tube current by adjusting the electric current flowing through the filament and thereby adjusts the characteristics of the X-rays. However, it takes time for the temperature of the filament to change, so that it takes time to change the tube current, and therefore it takes time until a set X-ray condition (tube current) is achieved.